In recent years, driving assistance apparatuses (called “driving assistance applications” hereinafter) that assist in the driving of a moving vehicle are being developed. For example, lane keep apparatuses that automatically steer or assist in driving so that vehicles travel along a road, lane departure prevention apparatuses that assist in driving by preventing vehicles from departing from their lanes, emergency avoidance apparatuses that automatically steer vehicles in order to avoid obstructions on the traveled road surface, and so on are being developed.
Request signals outputted from a driving assistance application such as a lane keep apparatus, a lane departure prevention apparatus, or an emergency avoidance apparatus (for example, a signal indicating a target lateral acceleration) are inputted into a lateral motion control apparatus that controls the amount of lateral motion (for example, the yaw rate) of the vehicle. Control signals are outputted from this control apparatus to a control target such as an actuator. The lateral motion of the vehicle is controlled as a result of the control target being controlled based on the control signals.
In the case where a driver has performed steering operations while the lateral motion of the vehicle is being controlled by the lateral motion control apparatus based on a request signal outputted from a driving assistance application (such interventional steering performed by the driver will be called “overriding”), the lateral motion control performed by the lateral motion control apparatus is stopped in order to give priority to the steering operations performed by the driver.
JP2006-117181A discloses a vehicle steering control apparatus that, when an electrical current instruction value for assistance control based on a steering torque has exceeded a determination reference value during an automatic steering mode, switches its steering mode from the automatic steering mode to a manual steering mode. Meanwhile, JP1998(H10)-278823A discloses a vehicle steering control apparatus that determines whether or not a present situation is a preparatory state for the start of steering control based on a preparatory operation performed by a driver and the travel conditions at that time. When the situation is the preparatory state, the vehicle steering control apparatus then determines whether or not the present situation is a starting state for steering control based on a starting operation performed by the driver and the travel conditions at that time. And when the situation is the starting state, the vehicle steering control apparatus commences the steering control.